The specific, normal acoustic impedance ("acoustic impedance" herein) of a material gives information concerning the acoustic absorption and reflection of the material. Such knowledge is important in the noise control of gas turbine aircraft engines. Further, since the acoustic impedance is a function of the structure and composition of the material, knowledge of the acoustic impedance can be used in quality control testing.
FIG. 1 illustrates a prior art device 2, called an impedance tube, for measuring the acoustic impedance of a material. The device includes a cylindrical tube 3 which contains at one end an acoustic source 6 which generates planar sound waves 9. A piece 12 of the material to be tested is cut to the same diameter of the tube 3 and is inserted into the tube 3 so that a tight fit results at interfaces 15A, 15B, and 15C. An acoustically hard (that is, sound reflecting) plate 18 is fastened to the tube 3 behind the piece of material 12 and functions as a back plane. The acoustic source 6 projects incoming plane waves 9 of acoustic energy to the material 12. The material reflects some of the acoustic energy at surface 21 and the rest of the energy travels through the material 12 and is partially absorbed by the material. The plane wave energy reaching the back plane 18 is reflected toward the acoustic source 6. The waves reflected at the surface 21 and at the back plane 18 interact with incoming waves to form a standing wave pattern (not shown) in region 24 of the impedance tube.
The reflection coefficient of the material 12 at a given frequency can be calculated from the standing wave pattern in a known manner based on the measurements made by two acoustic transducers 27 and 29 together with knowledge of the distances 32 and 34, which are the distances separating the transducers 27 and 29 from the material 12, and knowledge of the speed of sound which is calculated from the temperature of the air in the tube. The acoustic impedance of the material 12 is readily derived from the reflection coefficient. Further information concerning such impendance computation is given in the article entitled, "Error Analysis of Spectral Estimates with Application to the Measurement of Acoustic Parameters using Random Sound Fields in Ducts," by A. F. Seybert and B. Soenarko, appearing in the Journal of the Acoustic Society of America, Vol. 69, No. 4, April 1981, which is hereby incorporated by reference.
The use of the impedance tube 2 in FIG. 1 suffers the disadvantage of employing a destructive method of testing. That is, the method is destructive in the sense that the piece of material 12 must be severed from a component whose impedance is to be measured in order to fit into the tube, thereby inflicting damage upon the component. In the case of aircraft components, the monetary cost of such damage is high.